The present invention relates to closed cycle cryogenic coolers. More particularly, the present invention relates to an improved piston and spring assembly for use in such cryogenic coolers.
Closed cycle cryogenic coolers (hereinafter “CCCCs”) are known, an example of which may be seen in commonly owned U.S. Pat. No. 5,822,994, the entire disclosure of which is incorporated herein by reference. FIG. 1 of the '994 patent is reproduced as FIG. 1 in the instant application and is seen to include expander section 9 having a cylinder 11 and coldfinger assembly 18 within which is located a low friction linear clearance seal liner 15 bonded to a piston 13. The reciprocating movement of piston 13 is controlled in part by a spring/retainer assembly to control the axial motion of the expander. The principle cooling in a CCCC is accomplished through expansion of the helium working fluid. The expansion is made possible by achieving a phase shift between the pressure oscillation from the compressor 7 and the working fluid helium mass flow through the expander 9. The phase shift of the expander displacement is controlled using a spring/retainer configuration. The mechanical connection of the retainer to the expander or piston 13 is typically accomplished by matching the helical pitch of the spring to the other components and threading the assembly together.
FIG. 3 shows an expander section 50 wherein is located a prior art three piece assembly with a helical spring 20 threaded to a retainer 30 at one end thereof and a piston 40 at the opposite end thereof. A common problem with the traditional three piece spring, piston, and retainer designs used in CCCC applications is the inherent relative motion induced by the wound helical spring threading onto the piston and retainer. Any off-axis movement has the potential to cause debris due to surfaces rubbing against each other. An enlarged photo showing debris in the cylinder caused by piston and/or spring off-axis movement is shown in FIG. 2. Such debris degrades the useful life of the piston assembly and decreases operating efficiency of the CCCC.
Many alternative piston/spring approaches have been considered including the use of a Higby cut, rolled “light-bulb” threads, and electro-polishing to reduce the intrinsic debris generation with the current three piece design. None of these approaches have satisfactorily improved the debris generation problem.
There therefore remains a need for a retainer, spring and piston assembly for use in a CCCC which satisfactorily reduces the debris generation and degradation problem of the prior art designs.